The present invention relates to a spindle motor unit having a hydrodynamic bearing that can be used for a hard disk, and the like.
A conventional spindle motor unit having a hydrodynamic bearing is described below. FIG. 3 is a cross sectional view of a conventional spindle motor unit having a hydrodynamic bearing used for a hard disk. FIG. 4 is a cross sectional view, on an enlarged scale, of a portion of the spindle motor shown in FIG. 3. In FIGS. 3 and 4, base 1 has radial bearing 1a, radial bearing escape 1b, and thrust support 1c. Rotating shaft 2 that co-axially rotates with respect to base 1 is inserted in the radial bearing 1a. Flange 3 is secured to the lower end of rotating shaft 2 by machine screw 4. Flange 3 has recess 3a for housing the head of machine screw 4, and herringbone grooves 3b and 3c on the top and bottom faces of the flange, respectively.
Herringbone grooves 3b and 3c are configured as a pump-in type. This configuration of grooves pushes lubricant 6 with larger force in the direction of the center of rotation than in the outward direction so that centrifugal force caused by rotation will not leak lubricant 6 out. Plate 5 is secured to the bottom face of base 1 by a machine screw. Plate 5 has thrust support 5a in a position opposed to herringbone grooves 3c on flange 3. Lubricant 6 exists in the clearance between radial bearing 1a and rotating shaft 2, the clearance between thrust support 1c on base 1 and herringbone grooves 3b on flange 3, and the clearance between herringbone grooves 3c on flange 3 and thrust support 5a on plate 5. Hub 7 with magnetic disks 8 mounted thereon is fixed to rotating shaft 2.
In a spindle motor unit having a hydrodynamic bearing structured above, a rotating body including magnetic disks 8 and rotating shaft 2 floats up in a predetermined position during its rotation; thus such information as picture, audio, and text, is recorded into magnetic disks 8 or recorded information is reproduced from magnetic disks 8 through a magnetic head (not shown). However, when lubricant 6 existing in herringbone grooves 3b and 3c is reduced to an amount less than necessary, the rotating body does not floats up in the predetermined position and thus the magnetic head can not put information into magnetic disks 8 or reproduce it therefrom correctly. Therefore, lubricant 6 in the herringbone grooves must be kept to an amount not less than necessary even when rotating and stopping operations are frequently repeated. It is, therefore, an object of the present invention to provide a spindle motor unit having a hydrodynamic bearing in which a necessary amount of lubricant 6 is retained in the herringbone grooves even when rotating and stopping operations are frequently repeated.
In the spindle motor unit having the hydrodynamic bearing of the present invention, the inner diameter surface of the base opposed to the outer diameter surface of the flange has a stepped shape in which its center diameter is larger than its upper and lower diameters. In addition, the clearance formed between the upper and lower parts of this inner diameter surface and the outer diameter surface of the flange is reduced. This structure allows a necessary amount of lubricant to be retained in the herringbone grooves even when rotating and stopping operations are frequently repeated. Thus a spindle motor unit having a hydrodynamic bearing that ensures reliability as a thrust bearing and performs correct recording and reproducing operations can be obtained.